Thanks for the info! I do have the 1:10 barrel, looks like I'll just have to see how it works out!The shoulder washer kit that PMM sells is the exact same kit that's included with the SureFire muzzle devices. I received my SureFire muzzle brake (SFMB-762-5/8-24) today and can confirm it includes the chamfered washer plus an assortment of timing shims. Same as what's for sale in this link:
Check out the deal on Surefire Shim Kit, SF3P / SFMB / Warcomp-762, 5/8-24 at MGWwww.midwestgunworks.com
Is your Scar the original design with the stepped 1:12" barrel and 3-prong flash hider or the redesign with the non-stepped 1:10" barrel and muzzle brake? There is a ton of misinformation and dangerous advice floating around the internet on this so please read the instructions and do as it says. Both PMM and SureFire have decent videos on YouTube but they tend to gloss over the most critical step.
If you have the stepped 1:12" barrel:
On page 3 it says "Note: With SCAR-H, use supplied spacer with chamfer facing away from shoulder." This confuses people because every other rifle calls for the chamfer to be facing the shoulder to act as a relief for the small radius that always exists at the bottom of the threads. If this same orientation is used on the Scar then the washer will have very minimal surface contact or possibly clamp on the chamfered surface (the chamfer OD is very close in size to the barrel OD but I haven't taken measurements). Regardless, the correct method is to flip the washer around like the manual says.
SureFire should make this more explicit but they clearly haven't updated their manual since FN decided to change the dimensions of the barrel. I've seen a handful of people say they disregarded that advice because they thought it must be a misprint and that, to me, is a very bad idea.
If you have the non-stepped 1:10" barrel:
I'd imagine (but have not confirmed) the washer should be installed the standard way where the chamfer faces the shoulder. The chamfered washer may not even be necessary depending on the thread depth of the flash hider and threaded length of the barrel. You don't want the tip of the barrel threads to bottom out inside the flash hider of you'll be locating the muzzle device off the incorrect surface. I have not seen these in person so don't take my word for it.